1. Field of the Invention
The present invention relates to a semiconductor integrated circuit, and more specifically, a semiconductor integrated circuit for controlling a variable capacitance capacitor using Micro Electro Mechanical Systems (MEMS) (hereinafter, “MEMS-type variable capacitance capacitor”).
2. Description of the Related Art
MEMS-type variable capacitance capacitors have capacitor electrodes formed at relatively-movable portions of an MEMS configuration, and cause such movable portions to deform to physically change a distance between those two electrodes, thereby changing the capacitance therebetween (see, for example, JP2002-36197A).
In addition to the capacitor electrodes (main electrodes), for example, drive electrodes for deformation and capacitance change are also provided. As a voltage (deformation voltage Vact) is applied between such drive electrodes, a distance between the capacitor electrodes is reduced by Coulomb force, thus increasing the electrostatic capacitance of the capacitor. Conversely, as the voltage application between the drive electrodes is terminated, elastic force which restores the deformed movable portions acts, so that the capacitor electrodes return to the original positions, thus reducing the electrostatic capacitance of the capacitor to the original value.
When it is required to keep the electrostatic capacitance of the capacitor constant after the distance between the electrodes is reduced, it is necessary to keep applying a constant voltage (hold voltage Vhold) between the drive electrodes to maintain the distance of the capacitor electrodes constant. The hold voltage (Vhold) is lower than the deformation voltage (Vact) needed to reduce the distance between the electrodes.
In order to reduce the amount of charges trapped in an insulator sandwiched between the drive electrodes (to avoid a condition so-called “stiction”), to improve the reliability of the MEMS-type variable capacitance capacitors, and to reduce power consumption, the hold voltage Vhold is set as low as possible. Moreover, to return the distance between the electrodes to the original distance, the voltage between the drive electrodes is set back to 0 V.
According to such electrostatic actuators, a condition is changed over from a condition in which the deformation voltage Vact that is a high voltage is supplied to a condition in which the hold voltage Vhold that is lower than the deformation voltage is supplied. In this case, however, a large voltage is applied to a switching transistor used for such changeover, resulting in deterioration of the reliability due to aging.